This application claims the benefit of Japanese Application No. 2002-066854 filed Mar. 12, 2002.
The present invention relates to a radio frequency (RF) pulse tuning method and apparatus, and more particularly to a method and an apparatus for so controlling an RF pulse for excitation use to make the flip angle of spins identical with its target value.
In a magnetic resonance imaging (MRI) apparatus, the subject of imaging is brought into the internal space of its magnet system, i.e. an imaging space in which a magnetostatic field is formed, a gradient magnetic field and a high frequency magnetic field are applied to the subject, magnetic resonance signals are generated from spins excited therein, and an image is reconstructed on the basis of the signals so received.
A high frequency magnetic field is applied in the form of transmission of an RF pulse. To ensure appropriate excitation of spins, the RF pulse is controlled in advance of imaging. The control of the RF pulse is also known as RF pulse tuning.
In RF pulse tuning, transmission of the RF pulse is performed on a trial basis while consecutively varying the transmission gain of the RF transmission apparatus, i.e. the amplitude of the RF pulse. The reach of the flip angle of spins is checked on the basis of the magnetic resonance signal each time to figure out the optimal transmission gain.
The optimal transmission gain means a transmission gain that can make the flip angle of spins identical with its target value. As the optimal transmission gain differs from one subject of imaging to another, RF pulse tuning is performed for each subject. In the following description, RF pulse tuning may be sometimes referred to as simply tuning.
In order to ensure proper tuning for a subject having a large physique, the RF pulse width can be changed over to a large value. The pulse width is changed over according to the body weight of the subject. The weight datum is inputted in advance by the user of the MRI apparatus.
The upper limit of the pulse width is set to a considerably high level in order to ensure that tuning to a subject having a large physique can be accomplished without fail. In other words, the RF transmission apparatus is so configured as to permit such an output.
Such an RF transmission apparatus has a considerable surplus in capacity relative to the truly required output in actual imaging. For this reason, where a large pulse width is used, the RF pulse is often transmitted at a considerably lower gain than the maximum gain. This means that the capacity of the RF transmission apparatus is not effectively utilized.
An object of the present invention, therefore, is to realize a method and an apparatus for so performing RF pulse tuning as to enable the capacity of the RF transmission apparatus to be effectively utilized.
(1) An invention to solve the problem noted above from one point of view is a method for tuning an RF pulse for excitation use so as to make the flip angle of spins identical with a target value, characterized in that the pulse width of the RF pulse is consecutively increased from its predetermined initial value until the flip angle of spins become identical with the target value.
(2) An invention to solve the problem noted above from another point of view is an apparatus for tuning an RF pulse for excitation use so as to make the flip angle of spins identical with a target value, characterized in that it is provided with a pulse width adjusting means for consecutively increasing the pulse width of the RF pulse from its predetermined initial value until the flip angle of spins become identical with the target value.
The invention from the viewpoints stated in (1) and (2), since the pulse width of the RF pulse is consecutively increased from its predetermined initial value until the flip angle of spins become identical with the target value, the RF pulse is tuned so as to minimize the pulse width and to maximize the pulse amplitude. By using such an RF pulse for spin excitation, it is made possible to effectively utilize the capacity of the RF transmission apparatus.
(3) An invention to solve the problem noted above from another point of view is a method for tuning an RF pulse for excitation use so as to make the flip angle of spins identical with a target value, characterized in that the pulse width of the RF pulse is consecutively increased from its predetermined initial value until the flip angle of spins becomes identical with an intermediate target value, and the conditions of the RF pulse for making the flip angle of spins identical with a final target value which is smaller than the intermediate target value are calculated from the conditions of the RF pulse for making the flip angle of spins identical with the intermediate target value.
(4) An invention to solve the problem noted above from another point of view is an RF pulse tuning apparatus for tuning an RF pulse for excitation use so as to make the flip angle of spins identical with a target value, characterized in that it is provided with a pulse width adjusting means for consecutively increasing the pulse width of the RF pulse from its predetermined initial value until the flip angle of spins become identical with an intermediate target value, and a calculating means for calculating the conditions of the RF pulse for making the flip angle of spins identical with a final target value which is smaller than the intermediate target value from the conditions of the RF pulse for making the flip angle of spins identical with the intermediate target value.
The invention from the viewpoints stated in (3) and (4), since the pulse width of the RF pulse is consecutively increased from its predetermined initial value until the flip angle of spins become identical with an intermediate target value, the RF pulse is intermediately tuned so as to minimize the pulse width and to maximize the pulse amplitude.
Since the conditions of the RF pulse for making the flip angle of spins identical with a final target value which is smaller than the intermediate target value from the conditions of the RF pulse for making the flip angle of spins identical with the intermediate target value, there is obtained a final tuning pulse which would minimize the pulse width and maximize the pulse amplitude. By using such an RF pulse for spin excitation, it is made possible to effectively utilize the capacity of the RF transmission apparatus.
It is preferable for the aforementioned calculation to figure out the pulse width at the maximum adjustable amplitude of the RF pulse with a view to minimizing the width of the RF pulse.
If the flip angle fails to reach the target value even though the amplitude of the RF pulse is raised to the adjustable maximum, it is preferable with a view to reducing the length of time required for tuning to determine the next increment of the pulse width according to the extent of failure to attain the target.
It is preferable for the aforementioned target value to be 90xc2x0 with a view to 90xc2x0 pulse tuning.
It is preferable for the aforementioned target value to be 180xc2x0 with a view to 180xc2x0 pulse tuning.
According to the present invention, it is possible to realize a method and an apparatus for carrying out RF pulse tuning so as to effectively utilize the capacity of the RF transmission apparatus.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.